Baby Development

First Trimester - Conception to Week 13

Week 2

  • Fertilized ovum becomes implanted in the lining of the uterus.
  • Placenta development begins.

Week 4

  • Fetus has eyes, ears, nose, spine, digestive tract, and nervous system.
  • Heart begins as a beating tube.

Week 8

  • All organs have formed.
  • Heart is functioning.
  • Bones are forming.

Week 12

  • An ultrasound can detect the sex of the baby.
  • Teeth, fingernails, and toenails start to grow.
  • Kidneys begin to release urine into the fetus’s bladder.
  • The fetus begins to move around in the amniotic fluid.

Second Trimester - Week 13 to Week 16


Week 13

Head hair and lifted ears make the baby’s face begin to look more human.

Week 18 - 20

  • Stethoscope can detect the beating of the heart.
  • The baby’s movements may be felt by the mother.
  • Eyebrows and eyelashes appear.
  • A fine downy hair appears all over the baby’s body.
  • A protective coating of vernix, a creamy substance, covers the baby’s thin, shiny skin.
  • Teeth fully develop in the baby’s gums

Week 21 - 25

  • The baby is about half as long as it will be when it’s born.
  • Breathing movements begin.

Week 26

  • The mother may be able to feel the shape of the baby through her abdomen.
  • The baby’s eyes can open now.

Third Trimester - Week 26 to Week 40

Week 26

The baby has developed enough to survive outside the womb with special care. From this point on, the baby continues to grow, strengthen, and prepare for life in the world.

Week 32

  • Skin turns from red to pink and wrinkles smooth out as the baby builds fat stores.
  • The baby begins to taste and becomes aware of sounds outside the mother.
  • In boys, the testicles begin the descent into the scrotum.
  • The pupils in the baby’s eyes can react to light.

Week 36 - 40

  • The baby’s lean body gives way to plumpness.
  • The downy hair or lanugo that once covered the baby begins to disappear, though the vernix layer remains.
  • In girls, the labia majora has developed and in boys, the testes are now in the scrotum.